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In heterosporous seedless vascular plants, modified leaves called microsporophylls bear microsporangia containing many microsporocytes that undergo meiosis, each producing four microspores. Each microspore may develop into a male gametophyte consisting of a somewhat spherical antheridium within the microspore wall.
Plant spores are most obvious in the reproduction of ferns and mosses. However, they also exist in flowering plants where they develop hidden inside the flower. For example, the pollen grains of flowering plants develop out of microspores produced in the anthers. Reproductive spores grow into multicellular haploid individuals or sporelings.
The second more recent hypothesis is that spores were an early predecessor of land plants and formed during errors in the meiosis of algae, a hypothesized early ancestor of land plants. [18] Whether spores arose before or after land plants, their contributions to topics in fields like paleontology and plant phylogenetics have been useful. [18]
Flowering plants contain microsporangia in the anthers of stamens (typically four microsporangia per anther) and megasporangia inside ovules inside ovaries. In all seed plants, spores are produced by meiosis and develop into gametophytes while still inside the sporangium. The microspores become microgametophytes (pollen).
[10] [6] Differing sized spores have been observed in many fossilized plant species. [7] For example, the species Lepidophloios, also known as the scale tree, has been shown in fossils to have been heterosporous; [6] The scale tree had separate cones containing either male or female spores on the same plant. [6]
Microsporangia occur in all vascular plants that have heterosporic life cycles, such as seed plants, spike mosses and the aquatic fern genus Azolla. In gymnosperms and angiosperm anthers, the microsporangia produce microsporocytes, the microspore mother cells, which then produce four microspores through the process of meiosis.
The term "spore" is used to describe a structure related to propagation and dispersal. Zygomycete spores can be formed through both sexual and asexual means. Before germination the spore is in a dormant state. During this period, the metabolic rate is very low and it may last from a few hours to many years.
Microsporidia produce highly resistant spores, capable of surviving outside their host for up to several years. Spore morphology is useful in distinguishing between different species. Spores of most species are oval or pyriform, but rod-shaped or spherical spores are not unusual. A few genera produce spores of unique shape for the genus.